Sex Determination

Sex determination in higher animals is controlled by a specific pair of chromosomes.

For instance, in human beings, there are 46 chromosomes (23 pairs of homologous chromosomes) in every body cell. The genes that determine whether a child becomes male or female are located on the specific pair of chromosomes called sex chromosomes which are of two types, X and Y chromosomes (named after their shape).

The remaining 22 pairs of chromosomes are called autosomes and are responsible for other inheritable characteristics.

A male human being carries the XY (heterogametic) combination while a female carries XX (homogametic) combination.

After meiosis in a male, the spermatozoa can either contain the X or Y chromosome while the female ova will contain only X chromosomes. The sex of a child is a matter of chance and depends only on whether a spermatozoan that fertilizes the ovum carries X or Y chromosome.

During fertilization, there are four possible genotypic combinations. Two out of these four combinations (XX) results in a girl while the other two combinations (XY) result in a boy. There is therefore a 50% chance that Fertilisation can result in either XX (girl) or XY (boy).

In terms of probability, the chance that a boy or girl is produced in a family is a half (1/2). This means that chances are equally likely that a boy or girl is born in a family.

In other animals, the male is homogametic, while the female is heterogametic.

In birds, the male is XX and the female is XY.

In some insects, the female is XX and the male is XO; the Y chromosome is missing altogether.

In the fruit fly, Drosophila melanogaster, sex determination is exactly as in human beings, that is, males are XY and females are XX.